WELCOME!! AWWwww, what a CUTIE you have there!!

how adorable, what did you name him? I much admire the GSD, so much, such magnificent dogs, imo.
AND YAY FOR YOU for wanting to train your dog! Yes! I like you already, i wish everyone would give their dogs a chance to learn things.
Of course, teaching an infant baby dog what you do want, will take longer,
and take more patience on your part,
than it would for an adult dog, but, it can be done.
He sounds like a typical puppy, sure does!
If you are NEW to dog training,
remember, it's usually better and easier, to teach/reward what you DO want,
instead of trying to correct what you don't want.<--often this is useless anyway.
And of course, no one should ever hit a dog, yell at a dog, yank their leash, etc, (sorry, i don't know you, i'm sure you already knew that, but, just in case)
Here are some ideas to help teach your adorable infant baby dog how to control that natural urge all puppies have to mouth everything----here Kikopup is demonstrating how to reward what you DO want.
Dogs notice,
remember,
and tend to repeat behaviors they get rewarded for.
^NOT every trainer on youtube,

can be trusted to give good advice,
but that is KIKOPUP, and i can strongly recommend ANY of her videos-----Kikopup is
"positive only" which is best way to teach a dog anything you want to teach him.